1. Field of the Invention
This invention relates generally to microelectronic semiconductor structures and more particularly to improved monolithic photovoltaic semiconductor devices.
2. Description of the Prior Art
Technological advances on a plurality of diverse fronts have created a need for compact, efficient and highly reliable photovoltaic sources operable over indefinite extended periods of time. While early requirements for such devices were stimulated primarily by military and space-oriented programs, more recent commercialization of microelectronic batch fabrication technology has had a significant impact in all areas of commercial electronics, creating a demand and ready market for low-cost but efficient and reliable photovoltaic sources. A particular need has been created for such power sources in "personalized" electronic assemblies such as wrist watches, camera light sensing and shutter control applications, hand calculators and the like, for powering surgically implantable electronic devices such as heart and nerve stimulators, where it is undesirable to have to periodically surgically replace spent batteries of conventional design, and for a number of improvements in electromechanical instrumentation. on a larger scale, the low efficiencies inherent in the indirect processes for converting mechanical or chemical to electrical energy, coupled with decreasing supplies of natural resources conventionally employed for power generation have stimulated research for developing practical and inexpensive means for efficiently directly tapping, on a large scale, the vast reserves of solar energy for electrical power generation.
It has been only within the last several decades, with the advent of modern microelectronic semiconductor technology, that man has succeeded in directly converting solar to electrical energy on a meaningful basis via photovoltaic semiconductor cells and batteries. One of the earliest workable semiconductor solar batteries was constructed by forming a large area photosensitive junction in a block of silicon by solidphase diffusion techniques and by providing a conductive output path for the electrical current generated thereby by simply making electrical contacts to the two opposite sides of the block. This structure provided a more efficient photovoltaic device than any previously available. Subsequently, considerable effort has been invested in studying and refining both semiconductor photovoltaic cells and batteries constructed therefrom.
A physical limitation with known silicon microelectronic photovoltaic devices for generating electric current is the fact that while the output current of an individual photovoltaic cell varies proportionately with the active photovoltaic junction area of the cell, the voltage across any given cell is relatively fixed at a low value of approximately 0.5 to 0.6 volts per cell. Therefore, to configure a photovoltaic semiconductor battery having an output voltage in a usable voltage range, it has generally been recognized that a plurality of such individual photovoltaic cells must be connected in series to produce the desired output voltage.
Historically, the development of photovoltaic semiconductor devices has primarily been directed with one of two diverse principles in mind: (1) to construct via state-of-the-art semiconductor technology a more efficient individual photovoltaic cell, or (2) to configure a manageable and reliable interconnection scheme for individual solar cells, of whatever design, into series and parallel connected arrays of such cells for forming high-voltage batteries therefrom. Relatively little has been done to structure an inexpensive high-efficiency and reliable photovoltaic cell having a simple topology whose configuraton is completely compatible with microelectronic batch fabrication processes, such that a monolithic high-voltage photovoltaic array of such cells can be formed in those same simple processing operations in which the individual photovoltaic cells themselves are produced.
The operative structure required by most of the prior art individual photovoltaic cell configuratons does not permit of such cells being interconnected in one simple processing step while still in the wafer stage of processing. Most of the prior art cells require, for maximum efficiency, that electrode contacts be made to two opposite sides of the bulk material forming the cell. Such structures do not lend themselves as readily to standard microcircuit processing interconnection techniques, but require additional low-yield handling and processing which drives the cost of such devices up. In addition, hybrid (thin/thick) film techniques are generally required to interconnect such cells into series connected arrays, further decreasing yields and increasing the expense of such devices. Those configurations which do provide one-surface electrode contacts for the cell typically do not maximize their efficiency, since such one-surface contacts are typically provided at the expense of active photovoltaic junction area of the cell. Other individual cell configurations which attempt to maximize cell efficiency, for example by orienting the active junctions of the cell for illumination at their edges, require specialized base materials or fabrication processes which are not directly compatible with those employed for the fabrication of standard digital or analog integrated circuit components. Therefore, such devices typically cannot be integrated into the same wafer material as the active components which they are to energize. Further, the specialized structure of most prior art photovoltaic cell configurations dictates that they rely upon a single pass through a single junction to generate photovoltaic current, thus not permitting the simultaneous use of a plurality of features therein for increasing the cell efficiency by such techniques as multiple-pass internal reflections of the received solar energy, by inversion layers for capturing blue light energy, and the like.
A relatively small minority of prior art photovoltaic semiconductors have been designed with the intent of configuring monolithic arrays of photovoltaic cells, while still in the wafer form through batch fabrication operations, and while attempting to maximize efficiency thereof. However, for the most part, such devices are of highly specialized construction, requiring uncommon processing steps which are generally not directly compatible with the fabrication of other monolithic active devices upon the same wafer material.
Several of the prior art arrays of photovoltaic semiconductor cells use reverse biased p-n junctions for isolating individual cells therein from one another. Junction isolation techniques work well with standard integrated circuit components, where great care is taken to keep the isolating junctions of the integrated circuit in a "dark" environment. However, such isolation junctions when used in photovoltaic semiconductor devices, which are directly exposed to the solar activating energy, actually decrease the efficiency of the array since the leakage of the isolating junctions increases when exposed to energetic electromagnetic radiation. With the use of junction isolation techniques, the integrity of the isolating junction becomes extremely important, since that junction must be called upon to withstand the entire array voltage and any leakages therein can be disastrous.
The present invention overcomes these disadvantages of the prior art semiconductor photovoltaic cells and arrays by providing a topologically simple, efficient, highly reliable and low-cost photovoltaic cell configuration fabricated by standard microelectronic batch fabrication techniques which are completely compatible with those processes for fabricating standard integrated circuit possibilities if one chooses to place the photovoltaic devices on the same piece of silicon with the other active components. One embodiment of the individual photovoltaic cell configuration of this invention employs dielectric isolation techniques, for enabling, while still in wafer form, the simultaneous fabrication, isolation, and interconnection in series of individual photovoltaic cells through the use of simple, short, batch-fabrication processing stages.
While the invention will be disclosed with respect to several embodiments thereof, illustrating preferred topological layouts and methods of fabricating the devices, it will be understood that concepts encompassed by this invention apply equally well to other topological configurations and fabrication techniques respectively. Further, while the invention will be described in connection with specified materials, it will be understood that the scope of this invention is broad enough to apply equally well to other known or unknown materials yielding the desired operational properties of the inventive photovoltaic devices. Further, while the invention will be described in connection with certain combinations of desired features, it will be understood that this invention applies with equal effect to other combinations thereof.
Cl SUMMARY OF THE INVENTION
The present invention provides an improved apparatus and method for constructing by means of standard microelectronic batch fabrication techniques photovoltaic cells and monolithic high-voltage arrays thereof which are completely compatible with those processes for fabricating standard integrated circuit active components. The photosensitive junction or junctions of semiconductor devices built according to the principles of this invention are formed within a thin layer of single-crystalline semiconductor material overlying a relatively thick supportive substrate body. The thin layer of single-crystalline semiconductor material has at least one broad surface suitable for receiving energetic electromagnetic radiation, and at least one pair of sublayers of alternating first and second conductivity types forming at their intersection a p-n junction in a plane essentially parallel to the upper irradiated surface and substantially underlying the entire area thereof.
In a first embodiment of the invention, a relatively thin layer of dielectric insulating material such as silicon dioxide underlies the upper thin layer of single-crystalline material. The dielectric insulating layer is in turn supported by a relatively thick layer of non-monocrystalline semiconductor material. In a second embodiment of the invention, the supportive substrate comprises a relatively thick layer of transparent single-crystalline insulating material such as sapphire or magnesium aluminum spinel. In a third embodiment of the invention, the supportive substrate comprises a relatively thick layer of single-crystalline material, the uppermost region of which defines the lowermost sublayer of the thin layer of singlecrystalline semiconductor material.
A pair of spaced elongate zones of heavily doped first and second conductivity types extending from the irradiated upper surface and into conductive engagement respectively with those sublayers of like conductivity type, provide low-impedance current paths for majority carriers from the respective contacted sublayers and comprise the electrodes for a photovoltaic cell.
A plurality of features may routinely be incorporated into the fundamental structures of the photovoltaic cell for increasing the cell's efficiency. These features include, but are not limited to: inversion layer means at the upper irradiated surface of the thin layer of single-crystalline semiconductor material for producing photocurrent at the irradiated surface when exposed to blue light energy; a plurality of narrow projections extending from one or both of the elongate electrode zones toward the opposing electrode in interdigitated comb-like manner for decreasing the equivalent series resistance of the cell; a plurality of sub-layers within the thin single-crysalline semiconductor layer alternating by first and second conductivity types to form a plurality of photosensitive p-n junctions within the thin layer of single-crystalline semiconductor material, which junctions are essentially parallel to the upper irradiated surface; a high-low junction (to be hereinafter described) underlying the lowermost photosentive p-n junction within the thin single crystalline semiconductor layer for providing an electric field for urging minority carriers toward the overlying photosensitive p-n junction(s) and for enhancing lateral conductivity; for the first embodiment structure characterized by a non-monocrystalline substrate with a thin overlying dielectric layer--a reflective medium disposed either between the dielectric layer and the underlying non-monocrystalline substrate material or between the dielectric layer and the overlying thin single-crystalline semiconductor for reflecting energetic electromagnetic radiation back into the thin single-crystalline semiconductor material; and for the second embodiment structure characterized by a transparent single-crystalline substrate material--reflective media within the substrate material or at the lower surface thereof for reflecting energetic electromagnetic radiation back into the overlying thin layer of single-crystalline semiconductor material.
Means for providing a multiple-pass environment may be included in cooperation with any of the reflective layer features for causing total internal reflection of energetic electromagnetic radiation passing into the photovoltaic device through the upper irradiated surface.
Monolithic high-voltage photovoltaic cell arrays are fabricated by using the same processing steps employed to structure single photovoltaic cells, wherein those embodiments of the invention which use insulating substrate and dielectric layer materials are employed. Electrical isolation of individual cells within the arrays may be achieved either by means of appropriately disposed standard diffusions or by means of shaped dielectric barriers or moats.
For those monolithic photovoltaic array embodiments which achieve inter-cell isolation by means of diffusion techniques, a plurality of first and second heavily doped conductivity type elongate zones, aligned generally parallel to one another and successively alternating between those of the first and the second conductivity types are oriented intermediate and generally parallel to the electrode zones and extend continuously from the irradiated upper surface into conductive engagement with those respective sublayers within the thin single-crystalline semiconductor material having a like conductivity type. The "intermediate" elongate zones are disposed in closely adjacent pairs of opposite conductivity types and extend entirely across the thin layer of semiconductor material which is to define the photovoltaic cells. That one of the intermediate elongate zones of each closely adjacent pair which has a conductivity type opposite to the lowermost sublayer of the thin layer of semiconductor material, extends entirely through the thin layer and into engagement with the underlying insulating substrate or dielectric layer. Since the heavily doped conductivity zones comprising the widely spaced and the intermediate zones, and which are of the same conductivity type, can be simultaneously diffused, no additional processing steps need be required to fabricate the photovoltaic arrays than are required to fabricate individual photovoltaic cells.
Each portion of the thin layer of single-crystalline semiconductor material that is disposed between successive widely spaced pairs of the closely adjacent zones forms a single photovoltaic cell of the array with the closely adjacent pairs of elongate zones, in combination with the underlying insulating layer cooperatively providing isolation between adjacent cells. The closely adjacent intermediate elongate zones can be oriented to just touch one another or can be spaced slightly apart from one another, in which cases an overlying metallization strip provides series connection between the adjacent cells. Alternatively, the closely adjacent elongate zones can be disposed to substantially overlap one another along their lengths, simultaneously providing with said underlying insulating layer, isolation of adjacent cells while establishing electrical series connection thereof.
Formation of dielectrically isolated monolithic photovoltaic arrays can also be achieved within the scope of this invention by electrically isolating adjacent cells directly by means of shaped dielectric barriers or moats. Dielectric insulating barriers are fabricated by means of standard monolithic batch fabrication techniques so as to continuously project upwardly from the underlying insulating substrate and entirely through the thin upper layer of single-crystalline semiconductor material, to electrically isolate those portions of the thin upper layer which lie on oppositely disposed sides respectively thereof. The dielectric barriers may be oriented so as to completely surround, in moat-like manner, individual photovoltaic cells, or may be disposed to laterally extend entirely across the thin layer of single-crystalline semiconductor material to longitudinally subdivide adjacent portions thereof into a plurality of photovoltaic cells that are electrically isolated from one another. The spaced zones of heavily doped first and second conductivity types are diffused through the upper irradiated surface and into the thin single-crystalline semiconductor material, as previously described, forming electrodes of the isolated cells. The electrode diffusions of adjacent cells of the array can be disposed closely adjacent one another on opposite sides of the cell separating dielectric barriers. The electrodes of adjacent cells can be interconnected by metallization patterns formed by means of standard monolithic batch fabrication techniques on the upper irradiated surface of the array, so as to selectively bridge the dielectric barriers for electrically connecting the isolated cells in series or parallel as desired.
Those efficiency-improving features previously described with respect to the fabrication of individual cells may readily be incorporated into the batch fabrication construction of photovoltaic arrays constructed according to the principles of this invention, regardless of the specific inter-cell isolation technique used.